The present invention relates to a magnetic disk device, and more particularly to a magnetic disk device, which is suitable for positioning a magnetic head for recording and reproducing data, to a target track.
The magnetic disk devices are used as external storages for computers. In the magnetic disk device, a magnetic head is moved to a target track on a rotating magnetic disk to record and reproduce data on it. To realize a high track density of magnetic disk devices, it is essential to reduce the amplitude of the vibrations of vibration sources and improve the resistance to disturbances. The vibrations synchronous with the rotation of the disk (synchronous vibration: RRO) are chiefly the disk vibration when position information for servo was written (servo track write: STW) and the head actuator vibration written on the disk, and the vibration frequencies of large amplitude occur in the neighborhood of 2 kHz, which are a frequency band where the suppression performance of the servo system is lowest.
In other words, when the magnetic disk is used as storage, position information for servo is written on the disk by using an external actuator, or a laser length measuring system. In this servo track write operation, position information is written accompanied by vibrations of the disk and the head actuator, so that the position information includes values that deviate from a truly circular track (a locus of the head that should desirably be).
U.S. Pat. No. 6,097,565 discloses a technique to compensate for the synchronous vibration (RRO) to reduce the effects of the synchronous vibration, which comprises estimating positional displacements from a truly circular track, included in the servo signal written on the disk, by an arithmetic operation using an inverse characteristic of the servo characteristic (an inverse sensitivity function), adding an estimate, with its sign inverted, to the measured position to compensate for the synchronous vibration (RRO) to change the target position signal, which the head follows, to a truly circular track.
More specifically, in a control system for generating a position error (PES) between a head position (POS) and a target track (RRO), a control input (DAC) by a servo control unit (C) is obtained based on the above-mentioned position error data, and the head is positioned by driving the head actuator (P) according to the control input, position error data (PES) is collected as the position error signal on a sector-to-sector basis, the collected position error signal data is averaged on a sector-to-sector basis to obtain position-error component (RPES) synchronous with the rotation of the disk, and convolution integration with respect to the above-mentioned component is carried out by using an inverse sensitivity function (a transfer characteristic formed by adding 1 to an open-loop transfer characteristic Po•C (Po denotes a model of the head actuator and C denotes the servo control unit)), to obtain an estimate of the synchronous vibration. The synchronous vibration estimate is recorded as additional data near the position recorded on the disk, and when data is recorded or reproduced, the synchronous vibration estimate is reproduced together with servo information, and a reproduced synchronous vibration estimate is added, with its sign inverted, to the position error signal (PES), a deviation of the synchronous vibration estimate from the position error signal is obtained as a controlled amount after the synchronous vibration is compensated, a control input (DAC) is obtained based on the controlled amount, and therefore by driving the head actuator by the controlled amount, synchronous vibration compensation is implemented to change the target position signal so that the head follows a truly circular track on the disk.
In the prior art, in calculation of the synchronous vibration estimates, it has been a general practice to execute a process to extract rotation-synchronous component data from the position error signal and also execute a process of convolution-integrating an inverse sensitivity function with respect to the extracted rotation-synchronous component data. Therefore, in learning synchronous vibration to extract rotation-synchronous component data, it is considered important from a viewpoint of cost reduction in the manufacturing facility to finish this learning operation in as short a time as possible without deteriorating the compensation performance. To this end, it is most effective to reduce the number of times of execution of the averaging process to extract synchronous vibration data during the learning operation. However, the reduction of the number of times of execution of the averaging process causes the asynchronous component not to be compressed sufficiently, and the asynchronous components are superposed on the synchronous vibration components.
In other words, the position error signal (PES) contains the rotation-synchronous vibration component and the asynchronous vibration component, and if the number of times of carrying out the averaging process of the position error signal is increased to ten times or more, for example, the asynchronous vibration component approaches 0, but if the number of times of execution of the averaging process is decreased to not more than three, the asynchronous vibration component is not compressed but superposed on the synchronous vibration component after the averaging process of the position error signal. Particularly, when the number of times of execution of the averaging process is small, the asynchronous vibration component increases at low frequencies, resulting in a decrease in precision of the synchronous vibration estimates, and the synchronous vibration component becomes erroneous. The error compression characteristic (1/(1+PC)) of the servo system is large especially in vibration component data of the first to third rotations or so. This leads to follow the misestimated synchronous vibration component data in an early few of rotations, and crosstalks occur between adjacent tracks.
As disclosed in JP-A-7-98948, it may be possible to perform a filtering process on generating synchronous vibration component in real-time, but if the pre-filtering process is not applied to estimate the synchronous vibration component, learned values with high precision can not be obtained.